Active noise cancellation is a technique where undesirable noise is reduced or eliminated. This is typically done by detecting noise signals and generating inverted signals that counteract the noise signals. The inverted signals ideally have the same amplitude and an inverted phase as the detected noise signals. In practice, the inverted signals approximately match the amplitude and phase of the noise signals, and some noise still remains but at a substantially reduced level.
Some conventional noise cancellation techniques involve the use of Tow-Thomas biquad filters, which use three amplifiers (typically implemented within an integrated circuit chip). This architecture uses two capacitors in a single-ended design and four capacitors in a fully-differential design. For low-frequency/low-noise applications, the capacitors are typically external to the integrated circuit chip. Because of the standard Tow-Thomas design, an integrated circuit chip needs eight input/output pins in order to support fully-differential Tow-Thomas biquad filters. In a stereo architecture with two audio channels, eight capacitors and sixteen input/output pins would be needed. The large number of input/output pins increases the size and cost of the integrated circuit chips.